As a technique for converting a pre-set energy source to an electrical energy and for capturing the so-converted electrical energy, thermal power generation and nuclear power generation are being routinely used. In the thermal power generation, fossil fuels, such as coal or petroleum, is combusted to generate an energy which is converted through a mechanical energy into an electrical energy. In nuclear power station, a nuclear fuel is used to produce a nuclear fission and the nuclear energy so produced is converted into an electrical energy.
However, in thermal power generation, such problems as global warming due to carbon dioxide generated on combusting the fossil fuel are presented. On the other hand, the nuclear power generation is accompanied perpetually by a problem such as environmental pollution by radioactivity emitted in the reaction of nuclear fission and adverse effects on the human health.
That is, if the electrical energy is to be obtained by exploiting the above-described power generation technique, there is accompanied the effect on the environment, with the consequence that sustained dependence on the fossil fuel and on the nuclear energy poses a serious problem.
Meanwhile, a solar cell, as a photoelectric conversion device for converting the solar light into an electrical energy, has the solar light as an energy source, so that it has less adverse effect on the global environment, so that it is expected to be used extensively as future power generating device.
First, the efficiency of the solar cell, retained to be promising as next-generation power generating device, is explained with reference to FIG. 1. The solar cell is a device for converting the incident optical energy into an electrical energy. In the current technical level, a major part of the incident optical energy is lost in the course of the conversion process of the light energy to the electrical energy, and of extracting the electrical energy. Among the losses of the light energy incident on the solar cell, there are quantum loss, loss due to carrier recombination, surface reflection loss, absorption by the doping layer and loss due to serial resistance.
The ratio of the optical energy incident on the solar cell to the utilizable electrical energy corresponding to the incident optical energy minus the above losses is termed the effective efficiency of the solar cell. It is a general task in the preparation of the solar cell how to raise the effective efficiency of the solar cell.
Meanwhile, the basic structure of the solar cell is a diode having a cathode and an anode in a junction of p-and n-type semiconductors. If light falls on this semiconductor, an electron-positive hole pair is generated per photon in a level higher than the band gap proper to the semiconductor. The electron and the positive hole are isolated from each other by an electrical field of the pn junction and are drawn to the n-type semiconductor and to the p-type semiconductor, respectively, so that an electrical voltage (photo-electromotive force) is produced across both electrodes. If the cathode and the anode are connected to each other, an electrical power is produced.
Apart from the above-mentioned p-n type solar cell, having the pn junction, there is also a p-i-n type structure comprised of an i-layer of an intrinsic semiconductor area sandwiched between p-type and n-type semiconductor layers. The solar cell having the p-i-n type structure is wider in breadth than the p-type semiconductor layer or the n-type semiconductor layer. By the i-layer operating as a depletion area, the solar cell having the p-i-n type structure is able to absorb incident photons on the solar cell to generate as many electron-positive pole pairs as possible, as well as to make the light response faster. The above-mentioned i-layer becomes an optically active layer of the p-i-n type solar cell.
FIG. 2 shows an energy band diagram in case of short-circuiting the terminals of the p-i-n type solar cell. The Fermi level Ef in the p-type semiconductor layer is slightly above an upper end of the valence electron band, with the positive holes being majority carriers and with the electrons being the minority carriers. On the other hand, the Fermi level Ef in the n-type semiconductor layer is slightly below the conduction band, with the electrons being majority carriers and with the positive holes being the minority carriers. The i-layer, which is a junction of the p-type semiconductor layer and the n-type semiconductor layer, forms a potential barrier.
The material which constitutes the solar cell is generally silicon. The solar cell formed of silicon is roughly classified into a crystalline solar cell, formed of a single crystal solar cell or a polycrystalline silicon, and an amorphous silicon solar cell.
The crystalline silicon solar cell, which has so far been mainstream, is high lower at the current technical level than the amorphous silicon solar cell, suffers from the problem that, since crystal growth process is energy- and time-consuming, the solar cell is difficult to mass-produce, while being high in production cost.
Conversely, the amorphous silicon solar cell is lower in conversion efficiency in the current technical level than the crystalline silicon solar cell, however, it is higher in optical absorption, such that the thickness of the solar cell required for photoelectric conversion may be 1/100 of that of the crystalline silicon solar cell, and hence the solar cell can be constructed by depositing a layer of a thinner thickness. Moreover, the substrate material of the amorphous silicon solar cell may be selected from a wide variety of materials, such as glass, stainless steel, or polyimide-based plastic films, such that the amorphous silicon solar cell has such merits that it is broader in manufacturing tolerance and can be of an increased area. In addition, since the amorphous silicon solar cell can be reduced in production cost than the crystalline silicon solar cell, it is expected to be used in future in a wide range of fields of application from domestic use to a large-scale power generation plants.
The solar cell, as the smallest unit of the amorphous silicon solar cell can be prepared, as a result of development of the CVD (chemical vapor deposition) technique, by sequentially depositing semiconductor thin films having any desired composition or thickness. In general, a thin film of a phosphorus-containing n-type amorphous silicon, which is abbreviated below to a-Si:H, an impurity-free i-type a-Si:H thin film and a boron-containing p-type a-Si—H film are sequentially deposited on a substrate, such as glass substrate, to form a solar cell. This solar cell has a potential gradient from its surface receiving the incident light towards its back surface. It should be noted that a-Si:H is a hydrogenated amorphous silicon thin film into which hydrogen has been captured in forming the silicon thin film. By having hydrogen captured into the amorphous silicon, the light absorption coefficient in the visible light area can be increased to increase the light absorption coefficient in the visible light range. The conversion efficiency of the solar cell can be improved by employing this sort of the material as the battery material.
However, if the above-mentioned a-Si:H only is used in preparing the solar cell, the light having the wavelength not less than 800 nm can scarcely be used because the band gap of a-Si:H is on the order of 1.75 eV.
Thus, such a solar cell has been proposed in which the potential gradient is produced by impurities and two or more semiconductor materials having different band gaps are deposited in superposition to provide for efficient photoelectric conversion of the light beams of different wavelengths.
The solar cell having the above-described structure, termed a hetero junction type cell, has been proposed in view of the fact that the cell cannot photo-electrically convert the light lower in energy than the band gap of the semiconductor material forming the solar cell, and that the larger the band gap of the semiconductor material, the higher is the voltage that can be obtained by photoelectric conversion. With the hetero junction type solar cell, the photoelectric conversion efficiency is improved by providing plural semiconductor layers having band gaps corresponding to the incident light energy.
The hetero junction type solar cell aims at realizing effective light utilization by employing e.g., amorphous silicon germanium, termed below a-SiGe:H. However, this a-SiGe:H has a drawback that, although it exhibits more significant absorption to the light of longer wavelength and hence it is able to enlarge the shorting current, it exhibits an in-gap level formed in the gap higher than that with a-Si:H to decrease a curve factor to lower the conversion efficiency.
This problem is addressed by varying the composition of a-SiGe:H and a-Si:H to thereby vary the band gap continuously.
With this method, the closer the minimum value portion of the band gap of the i-layer to the p-type semiconductor layer as the light incident side, the optical deterioration may be lowered to improve the device reliability. This is due to the fact that the larger the distribution of the optical absorption in the vicinity of the p-type semiconductor layer, the higher becomes the degree of collection of the positive holes. However, there is raised a problem that, if the smallest value portion of the band gap is formed in the vicinity of the p-type semiconductor layer, the band gap of the i-layer in the vicinity of the p-type semiconductor layer becomes smaller to decrease the voltage value further. In addition, in this method, in which the band gap of the i-layer is decreased to increase optical absorption, the curve factor is increased with the band gap of the i-layer approximately 1.4 eV or less, thus imposing limitations in improving the conversion efficiency despite increased light optical absorption.
There is also proposed a method of providing an amorphous silicon carbide (a-SiC:H) layer, having a wide gap on the order of 2.1 eV, in an interface between the p-type semiconductor layer and the i-layer. However, this method suffers a problem that, since it is not possible to form an a-SiC:H layer of a high film quality, the optical deterioration, which may lead to the worsened hole movement following light irradiation, tends to be increased.
Meanwhile, in order to accommodate the solar cell to various usages, it is necessary to respond to the demand for achieving the lightness in the weight of the product, improved productivity, ease in machining the curved surfaces and for cost reduction.
The majority of the low melting materials or plastic materials can be molded to a desired shape at lower temperatures, so that machining costs can be advantageously decreased. Moreover, a plastics material has a merit that it is lightweight and less liable to cracking. Therefore, it is desirable to use the low melting material or plastics material as a substrate of the solar cell. If the plastic material, especially a general-purpose plastics, such as polyester film, as a substrate, it becomes possible to improve the productivity significantly using a roll-to-roll type manufacturing system employing an elongated substrate.
However, since the heat resisting temperature of the general-purpose plastics is generally 200° C. or lower, it is necessary to use a low-temperature film forming process and to realize film formation to a high film quality in the low-temperature film forming process.
If such materials as Si, Si1−xGex, Ge or Si1−xCx are subjected to a film-forming process at a substrate temperature of 200° C. or lower, the resulting film is usually amorphous. In the amorphous film, there exist a large number of elements, which serve as nuclei of re-combination of minority carriers, such as local energy level in the energy band gap, with the carrier length being shorter than that in a single-crystal film or a polycrystalline film.
Therefore, if an a-Si:H film, an a-Si1−xGex:H film, an a-Ge:H film or an a-Si1−xCx:H film, doped with an impurity, such as a film which has become a p- or n-type semiconductor layer on being doped with boron or phosphorus, is used as the p-type semiconductor layer and/or as the n-type semiconductor layer in the p-i-n type solar cell, the conversion efficiency is lowered due to the lower dark current ratio, thus proving a hindrance to the preparation of a high quality solar cell at lower temperatures. Therefore, if these materials are used, the dark current ratio must needs be 1×10−3 S/cm or less and moreover is required to be not less than 1×10−2 S/cm.
Moreover, in the p-i-n type solar cell, light absorption in the p-type semiconductor layer or in the n-type semiconductor layer does not contribute to improvement in the efficiency (so-called dead zone). The a-Si1−xGex:H film, an a-Ge:H film or an a-Si1−xCx:H film, doped with an impurity, is insufficient in the doping efficiency, so that the film tends to be depleted. If the a-Si1−xGex:H film, an a-Ge:H film or an a-Si1−xCx:H film is used, the film thickness needs to be thicker to a more or less extent to prevent the depletion. So, with this sort of the solar cell, in which the p-type semiconductor layer or the n-type semiconductor layer is increased in film thickness, the light absorption in these layers is increased to obstruct the improvement in the conversion efficiency.
Thus, in the p-i-n type amorphous silicon based solar cell, such a technique has been proposed in which only the p-type semiconductor layer doped with an impurity and the i-layer are crystallized to decrease the value of the light absorption coefficient as an index for ease in optical absorption to improve the conversion efficiency.
For example, in the Japanese Patent Publication H-6-5780, the p-type semiconductor layer and the n-type semiconductor layer of the hydrogenated amorphous silicon are irradiated with an excimer laser, whereas, in the Japanese Laying-Open Publication S-63-133578, the p-type semiconductor layer and the n-type semiconductor layer of the hydrogenated amorphous silicon are irradiated with the YAG-laser for annealing, thereby crystallizing the p-type semiconductor layer and the n-type semiconductor layer.
However, should the laser of an energy strength sufficient to crystallize the hydrogenated amorphous silicon be directly radiated on the film surface, the a-Si:H film is flown off under the pressure of precipitous hydrogen extraction from within the film, whilst hydrogen passivated below the poly-Si layer is also extracted to produce films of inferior optical properties having a large number of dangling bonds. FIG. 3 shows the state of the hydrogenated amorphous silicon film before and after irradiation of the excimer laser (ELA) of the aforementioned intensity.
It may be seen from FIG. 3 that hydrogen is ejected from within the hydrogenated amorphous silicon film as a result of laser irradiation. Should hydrogen be ejected from within the film, the film is destroyed.
In particular, in a film prepared at lower temperatures, there are many cases where a large quantity of hydrogen is contained between Si networks. In order to evade ablation resulting from sudden warming and resulting explosion of hydrogen, the operation of so-called hydrogen extraction by raising the temperature to approximately 400° C. in a furnace is required. FIG. 4 shows the manner in which a film is crystallized by irradiating excimer laser (ELA) on the film from which hydrogen is extracted as described above.
Should only the doping layer of the hydrogenated amorphous silicon be crystallized by annealing on laser irradiation, it is not possible to form a p-i-n type solar cell having a doping polycrystalline film of higher quality having only a smaller number of dangling bonds.